The mammalian immune system includes B lymphocytes that, in totality, express an antibody repertoire composed of hundreds of billions of different antibody specificities. A normal immune response to a particular antigen involves the selection from this repertoire of one or more antibodies that specifically bind the antigen, and the success of an immune response is based, at least in part, on the ability of these antibodies to specifically recognize (and ultimately eliminate) the stimulating antigen and “ignore” other molecules in the environment of the antibodies.
The usefulness of antibodies that specifically recognize one particular target antigen led to the development of monoclonal antibody technology. Standard hybridoma technology now allows for the preparation of antibodies having a single specificity for an antigen of interest. More recently, recombinant antibody techniques, such as screening of in vitro antibody libraries, have been developed. These techniques also allow for production of antibodies with a single specificity for an antigen of interest.
Antibodies having specificity for a single target antigen may, at least under certain circumstances, display undesired cross-reactivity or background binding to other antigens. This cross-reactivity or background binding, however, is usually unpredictable (i.e., it is not possible to predict which antigen the antibody will cross-react with). Moreover, it is usually distinguishable from specific antigen binding, since it typically represents only a very minor portion of the binding ability of the antibody (e.g., 1% or less of total antibody binding) and typically is only observed at high antibody concentrations (e.g., 1000 fold, or higher, concentrations than needed to observe specific antigen binding). Although there are several antigens that may belong to a structurally related family of proteins, the antibody response to a particular family member is highly specific. In addition, there are several examples of protein family members (e.g., members of the IL-1 and TNF families) that bind to the same receptor, receptor component or structurally related receptors, yet monoclonal antibodies raised against one member of the family do not show high cross reactivity towards other family members. There could be two reasons for this lack of cross-reactivity of MAbs towards various family members. First, in standard hybridoma production, one searches for only a few antibodies of high specificity/affinity for the target antigen and then checks for cross reactivity or background binding of the few selected antibodies. Second, although proteins within a family are structurally related, they may have exclusive, non-overlapping immunodominant epitopes. Therefore, MAbs raised by using full length protein may not cross react with other structurally related proteins.
There are also examples of monoclonal antibodies raised against an antigen of one species that will bind specifically to the same functional antigen in another species. For example, an anti-mouse X antibody may readily bind antigen X from human. This is because they share significant sequence and structural similarities though they are not identical. However, such species-cross-reactive antibodies do not constitute “dual specificity” antibodies, since they have specificity for the same antigen from different species.
Thus, monoclonal antibodies having predictable dual or multiple specificity, that is, antibodies having true specificity for two or more different antigens, are still needed.